In the manufacture of integrated circuits (ICs), a lithographic apparatus is used to apply a desired pattern onto a target portion of a substrate. Slit uniformity is a key factor for lithographic apparatuses. If the result exceeds or fails to meet the desired specification, the lithographic apparatus will take related actions to reduce the slit uniformity, e.g., adjusting the uniformity improvement package (UIP), uniformity correction module (UNICON) plate, or gradient filter replacement. After the slit uniformity optimization is performed, the lithographic apparatus will check the critical dimension uniformity (CDU) monitor value, and perform a spot sensor conversion factor (SSCF) compensation based on the CDU monitor result. To perform such SSCF compensation, the only reference value is the CDU monitor result. This is not a straight-forward strategy, any steps in the CDU monitor procedure flow can impact the SSCF compensation value. A wrong SSCF compensation value will negatively impact the product critical dimension (CD) seriously, so that it could result in wafer rework, or even wafer scrap.